An organic light emitting diode (“OLED”) device typically includes, for example: (1) an anode on a substrate; (2) a hole transport layer (“HTL”) on the anode; (3) an electron transporting and light emitting layer (“emissive layer”) on the HTL; and (4) a cathode on the emissive layer. When the device is forward biased, holes are injected from the anode into the HTL, and the electrons are injected from the cathode into the emissive layer. Both carriers are then transported towards the opposite electrode and allowed to recombine with each other in the device, the location of which is called the recombination zone.
FIG. 1 shows an energy level diagram for a prior art OLED device. In FIG. 1, the ionization potential (“IP”) is the energy difference between the vacuum level and the highest occupied molecular orbital (“HOMO”) level. The vacuum level is usually referred to as the reference level from which the energy levels are measured. The HOMO is the highest energy level filled with electrons and in which the holes are free to move. Similarly, the lowest unoccupied molecular orbital (“LUMO”) is the lowest energy level devoid of electrons and in which free electrons are free to move. The energy difference between the HOMO level and the LUMO level is the band-gap within which there are no available molecular orbital states. The IP value is a measure of the minimum energy, expressed in electron volts (“eV”), required to remove an electron from an atom. The IP for an anode comprised of indium tin oxide (“ITO”) is typically 4.8 eV. The IP for a HTL comprised of polyethylenedioxythiophene (“PEDOT”) and polystyrenesulfonic acid (“PSS”) (this is referred to, herein, as PEDOT:PSS) is typically 5.0 eV. The IP for an emissive layer comprised of blue emissive polymer material is typically from 5.8 eV to 6.0 eV. The work function for a cathode that includes an electron injecting layer comprised of, for example, barium or calcium is typically between 2.0 eV and 3.0 eV. The energy barrier is the difference between the energy levels of two adjacent layers. In this device configuration, there is usually a large energy barrier for hole injection at the interface between the HTL and the emissive layer that suppresses the injection of holes into the emissive layer. The energy barrier is considered large if, for example, there is a gap of 0.2 eV to 1.0 eV.
In this device configuration, the emissive layer is typically hole deficient. The hole deficiency can be caused by electrical or physical factors. For the former, the large energy barrier for hole injection at the interface between the HTL and the emissive layer suppresses the injection of holes into the emissive layer. For the latter, incompatibility between the emissive layer and the HTL may result in the emissive layer solution used to form the emissive layer not adequately wetting the surface of the HTL thus causing the coverage and adhesion of the emissive layer to the HTL to not be uniform and robust and this may suppress the injection of holes into the emissive layer. A deficiency of holes in the emissive layer results in reduced device efficiency since only a sub-optimal number of electrons are recombining. In addition, the lack of sufficient number of holes results in a shift of the recombination zone to the interface between the HTL and the emissive and this decreases the lifetime and efficiency of the device.
Therefore, in order to improve device efficiency and lifetime, the number of holes reaching the emissive layer should be increased.